A group of mutants at three distinct loci is being studied in order to investigate the regulation of the final expression of three enzymes, exanthine dehydrogenase, aldehyde oxidase and pyridoxal oxidase. The structurl genes incoding of the three enzymes are genetically distinct and the enzyes are physically separable from one another. Genetic and biochemical evidence argues that the loci of interest do not provide major structural elements to the enzymes they affect. Similarity of this system of mutants in Drosophila to the cnx mutants of Asperigillus indicates that the three loci of interest in Drsophila are indeed analogous to the cnx loci. This then implies that like the cnx loci, the Drosophila loci affect the synthesis and/or utilization of a molybdenum cofactor commonly required by the three enzymes. Work in progress seeks to understand the biochemical relationship between the three loci of interest and the cofactor in order to determine the role of this cofactor in affecting the final expression of three disparate enzymes.